This proposal focuses on the structure and function of protein complexes participating in the regulation of the myocyte enhancer factor-2 (MEF2) family of transcription factors. MEF2 serves as a molecular switch of specific gene expression in muscle cells, T cells and neuronal cells in response to calcium signals, playing pivotal roles in the development and adaptive responses of the muscle, immune and nervous systems. In heart muscle cells, deregulation of MEF2-controlled genes under stress and a variety of other pathological conditions has been linked to many forms of cardiovascular diseases. The long term goal of the proposed research is to build a high-resolution picture of the MEF2 pathway and analyze its mechanism and function using structure-based approaches. Specifically, X-ray crystallography, biochemical methods and cell-based assays will be used to investigate: (i) How MEF2 recruits transcriptional co-repressors including class II histone deacetylases to specific promoters and ensures proper gene silencing in resting cells (Aim 1);(ii) How MEF2/co-repressor complexes are disassembled by calcium-dependent mechanisms (Aim 2);(iii) How MEF2, upon the release of co-repressors, recruits transcriptional co-activator p300 to turn on specific gene expression (Aim 3). Small peptides that can bind MEF2 specifically and block the recruitment of co-repressors or co-activators will also be developed (Aim 4). The proposed studies in Aims 1-4 are based on the recent structural and biochemical characterization of the Cabin1/MEF2/DNA complex, which suggests that MEF2 possesses a signaling domain capable of binding to a variety of transcriptional co-regulators through related but distinct mechanisms. These studies will not only provide insights into the basic mechanisms of calcium/MEF2-mediated biological responses in a variety of cells but also help drug development in treating human heart diseases.